Distant Magnetosphere Research Articles

Does Uranus' Asymmetric Magnetic Field Produce a Relatively Weak Proton Radiation Belt?

AbstractSince the Voyager 2 flyby in 1986 the radiation belts of Uranus have presented a problem for physicists. The observations indicate the electron radiation belt is far more intense than the proton radiation belt, and while the electron intensities are close to the upper theoretical limit, proton intensities are well below. Here we propose the relatively weak proton radiation belt could be due to Uranus' asymmetric magnetic field. We model test particle motion through the field to show that perturbations arising from asymmetry are greater the larger the particle gyroradius, predominantly affecting 100‐keV protons. For these particles, more rapid changes in maximum distance from the planet during a bounce motion promote trajectory evolution into regions where they could be lost through impact with the rings, impact with the atmosphere, or to the distant magnetosphere and solar wind. We suggest this could explain a relatively weak proton radiation belt at Uranus.

Secular Drift of the Auroral Ovals: How Fast Do They Actually Move?

AbstractA surprisingly fast secular drift of the Northern geomagnetic dip pole during the last two decades has attracted much interest lately, in particular, evoking speculations about a possibility of a sweeping relocation of the auroral oval. This letter presents first results of a model investigation of this issue, based on an empirical representation of the distant magnetosphere combined with a series of internal geomagnetic field models for 12 epochs, covering the interval from 1965 to 2020. The secular drift of the northern auroral oval was found to result in its net displacement over the 55‐year period, commensurate with the concurrent shifts of the centered, eccentric, and corrected geomagnetic poles, all of them much smaller than the enormous spurt of the northern dip pole. In the Southern Hemisphere, the shift of the auroral oval and of the poles over the same period is much weaker, revealing a remarkable interhemispheric asymmetry.

Empirical Modeling of Dayside Magnetic Structures Associated With Polar Cusps

AbstractThe magnetic structure of dayside polar cusps is intimately related to the high‐latitude field‐aligned currents (FACs) and the cross‐B diamagnetic currents due to the penetrated plasma of magnetosheath origin. The dayside FAC configuration is sensitive to the azimuthal component of the interplanetary magnetic field, manifested in the latitudinal splitting and longitudinal overlapping of the Region 1 FAC zone around noon. The diamagnetism of the polar cusp plasma results in deep cleft‐shaped depressions of the ambient magnetic field. Neither of the above factors have yet been properly addressed in the existing empirical models of the distant magnetosphere. To fill this gap, an advanced data‐based model has been developed, including the spiral structure of the dayside FACs and their splitting/overlapping in response to the orientation and magnitude of the azimuthal interplanetary magnetic field component By. The cusp diamagnetic field depressions are modeled using a new approach, based on a 3‐D system of magnetic “bubbles” (Tsyganenko & Andreeva, 2018, https://doi.org/10.1029/2018GL078714). The magnetic effects associated with FACs and diamagnetic currents are represented within the framework of a single hybrid model, fitted to large subsets of spacecraft data. The obtained results are analyzed in the context of the dayside magnetic field response to interplanetary conditions and the geodipole tilt angle.

A hybrid approach to empirical magnetosphere modeling

AbstractA new approach has been devised and explored to reconstruct magnetospheric configurations, based on spacecraft data and a synthesis of two methods of modeling the magnetic field of extraterrestrial currents. The main idea is to combine within a single framework (1) a modular structure explicitly representing separate contributions to the total field from the magnetopause, ring, tail, and field‐aligned currents, and (2) a system of densely distributed field sources, modeled by the radial basis functions (RBF). In such an arrangement, the modular part takes on a role of the principal component representing the gross large‐scale structure of the magnetosphere, whereas the RBF part serves as a higher‐order correction that compensates for the lack of flexibility of the modular component. The approach has been tested on four subsets of spacecraft data, corresponding to four phases of a geomagnetic storm, and was shown to tangibly improve the model's performance. In particular, it allows proper representation of magnetic effects of the field‐aligned currents both at low altitudes and in the distant magnetosphere, as well as inclusion of extensive high‐latitude field depressions associated with diamagnetism of the polar cusp plasma, missing in earlier empirical models. It also helps to more accurately model the nightside magnetosphere, so that most of the large‐scale magnetotail field is compactly described by a dedicated module inherited from an earlier empirical model, while the RBF component's task is to resolve finer details in the inner magnetosphere.

Quantitative aspects of the Galperin <i>L</i> parameter

Abstract. A new geomagnetic parameter was suggested twenty years ago by Y. Galperin, the Galperin L parameter, and it was introduced into the CNES Maglib for French-Russian projects in the exploration of the distant magnetosphere. The definition and the advantages of the Galperin L parameter are recalled in this brief paper. Unforeseen possibilities in the use of this parameter for mathematical models of the magnetosphere are stressed using past results obtained with the Mead model. The Galperin L parameter is shown to add, in the synchronous region, a quantitative capability to the qualitative description (labelling) of the magnetosphere. More work will be necessary to adapt past mathematical models to present numerical models and extend the domain of the quantitative applications of the Galperin L parameter.

Structure of the auroral precipitation region in the dawn sector: relationship to convection reversal boundaries and field-aligned currents

Abstract. Abstract. Simultaneous DMSP F7 and Viking satellite measurements of the dawnside high-latitude auroral energy electron and ion precipitation show that the region of the low and middle altitude auroral precipitation consists of three characteristic plasma regimes. The recommendation of the IAGA Working Group IIF/III4 at the IAGA Assembly in Boulder, July 1995 to decouple the nomenclature of ionospheric populations from magnetospheric population is used for their notation. The most equatorial regime is the Diffuse Auroral Zone (DAZ) of diffuse spatially unstructured precipitating electrons. It is generated by the plasma injection to the inner magnetosphere in the nightside and the subsequent drift plasma to the dawnside around the Earth. Precipitating particles have a hard spectrum with typical energies of electrons and ions of more than 3 keV. In the DAZ, the ion pitch-angle distribution is anisotropic, with the peak near 90°. The next part is the Auroral Oval (AO), a structured electron regime which closely resembles the poleward portion of the night-side auroral oval. The typical electron energy is several keV, and the ion energy is up to 10 keV. Ion distributions are pre-dominantly isotropic. In some cases, this plasma regime may be absent in the pre-noon sector. Poleward of the Auroral Oval, there is the Soft Small Scale Luminosity (SSSL) regime. It is caused by structured electron and ion precipitation with typical electron energy of about 0.3 keV and ion energy of about 1 keV. The connection of these low-altitude regimes with plasma domains of the distant magnetosphere is discussed. For mapping of the plasma regimes to the equatorial plane of the magnetosphere, the empirical model by Tsyganenko (1995) and the conceptual model by Alexeev et al. (1996) are used. The DAZ is mapped along the magnetic field lines to the Remnant Layer (RL), which is located in the outer radiation belt region; the zone of structured electrons and isotropic ion precipitation (AO) is mapped to the dawn periphery of the Central Plasma Sheet (CPS); the soft small scale structured precipitation (SSSL) is mapped to the outer magnetosphere close to the magnetopause, i.e. the Low Latitude Boundary Layer (LLBL). In the near-noon sector, earthward fluxes of soft electrons, which cause the Diffuse Red Aurora (DRA), are observed. The ion energies decrease with increasing latitude. The plasma spectra of the DRA regime are analogous to the spectra of the Plasma Mantle (PM). In the dawn sector, the large-scale field-aligned currents flow into the ionosphere at the SSSL latitudes (Region 1) and flow out at the AO or DAZ latitudes (Region 2). In the dawn and dusk sectors, the large-scale Region 1 and Region 2 FAC generation occurs in different plasma domains of the distant magnetosphere. The dawn and dusk FAC connection to the traditional Region 1 and Region 2 has only formal character, as FAC generating in various magnetospheric plasma domains integrate in the same region (Region 1 or Region 2). In the SSSL, there is anti-sunward convection; in the DAZ and the AO, there is the sunward convection. At PM latitudes, the convection is controlled by the azimuthal IMF component (By ). It is suggested to extend the notation of the plasma pattern boundaries, as proposed by Newell et al. (1996), for the nightside sector of the auroral oval to the dawn sector.Key words. Magnetospheric physics (current systems; magnetospheric configuration and dynamics; plasma convection)

Variability of the electric currents in the magnetosphere

We used several thousand magnetic measurements in the distant magnetosphere to study differential responses of the magnetospheric currents to various geophysical parameters. It is shown that the growth of the solar wind dynamic pressure intensifies the magnetopause and magnetotail currents. The IMF vertical component affects the Region 1 field-aligned currents. The substorm activity gives rise to growth of the cross-tail current. The storm development is accompanied by the increase of both the ring and cross-tail current, the latter effect being dominant.

Jupiter's H +3 Emissions Viewed in Corrected Jovimagnetic Coordinates

The distribution of H + 3 emissions in Jupiter's polar region is interpreted in terms of the corresponding source regions and the energetic in the magnetosphere by reference to corrected jovimagnetic coordinates based on the VIP4 magnetic field model of J. E. P. Connerney et al. (1998, J. Geophys. Res. 103, 11929–11939). Two sets of 3.43-μm auroral images, acquired on May 31/June 1, 1996, and on July 8/July 9, 1996, using NSFCAM at NASA IRTF, were analyzed. Model images were generated for observed rotation phases, and the generalized inverse methodology of T. Satoh et al. (1996, Icarus 122, 1–23) is applied to pixel-by-pixel comparison of intensities to constrain the parameters of the emission source model. The most intense emissions are found between the 12- R j (jovian radii) and 30- R j ovals, representing the main auroral emissions. The System-III longitudes (λ III) of peak brightness in this zone, λ III ∼ 260° in the north and ∼130° in the south, coincide well with those of the minimum surface field magnitudes according to the VIP4 model. Between the 8- R j and the 12- R j ovals, the peak brightness occurs at λ III ∼ 215° in the north and at ∼ 25° in the south, consistent with the Satoh et al. (1996) System-III anomalies. These enhancements may originate from the “windshield wiper” effects of drifting electrons, caused by pitch angle scattering of varying efficiency from the inner to the distant magnetosphere. The total power emitted from the entire auroral region by the H + 3 ion at all ro-vibrational levels is estimated as 2.1×10 12 W in the north, and 1.2×10 12 W in the south. Relative contributions from the individual zones are ∼10% from the 6- to 8- R j zone, ∼20% from the 8- to 12- R j zone, ∼25% from the 12- to 30- R j zone, and ∼ 45% from the polar-cap emission, respectively.

Detailed study of FUV Jovian auroral features with the post‐COSTAR HST faint object camera

A set of Hubble Space Telescope faint object camera images taken in the H2 bands near 1550 Å is used to infer the morphological properties of the steady state Jovian FUV aurorae. We focus on issues best addressed using the excellent spatial resolution available after correction of the spherical aberration, i.e., those related to high latitude or small auroral features. A thorough comparison of the emission loci with model ovals highlights the improvement of the VIP4 magnetic field model over previous ones at all latitudes. The north‐south conjugacy of the main oval is now correctly accounted for, and second‐order discrepancies suggest non axially symmetric contributions of external origin. This oval is usually amazingly narrow (down to 80±50 km) and very bright, although quite variable with time (100 kR to 1–2 MR, i.e., peak input flux of ∼10–200 ergs cm−2 s−1). We discuss its structure, in latitude and longitude, and show that it is consistent with precipitation by pitch angle scattering. Fainter concentric narrow ovals are also present on the north polar cap, presumably at the footprint of open field lines. Both polar caps are partly covered by a faint diffuse emission, confined to the afternoon sector in magnetic local time. A bright extended feature, previously identified across the north polar cap along the 160° meridian, might be not a specific auroral feature but rather a region where inner arcs and diffuse polar cap emissions are intensified, maybe by a solar wind driven ionospheric process. Equatorward of the main oval, we identify a belt of moderate emission, attributed to a precipitation process distributed between the Io torus and the distant magnetosphere. Longitudinally confined bright areas lie in the same latitude range and consist of series of short segments of concentric arcs. We also present the discovery of a narrow faint oval at the footprint of Io's orbit. Finally, we confirm that the FUV footprints of the Io flux tube are very small (a few hundreds of kilometers or less), implying an interaction close to Io. The input energy flux in this spots is huge and variable (0.8–5 × 1011W).

Emission Source Model of Jupiter's H+3Aurorae: A Generalized Inverse Analysis of Images

We mapped the distribution of H+3emissions in Jupiter's polar regions utilizing 3.4-μm images acquired with ProtoCAM at the NASA IRTF. Two data sets, one acquired on February 28 and the other on March 22, 1992, were analyzed independently. A sequence of image mosaics was used to constrain the parameters of an emission source model. The emission source model utilizes an auroral oval traced from the distant magnetosphere, using the O6magnetic field model (Connerney, J. E. P. 1992. InPlanetary Radio Emissions III(H. O. Rucker, S. J. Bauer, and M. L. Kaiser, Eds.), pp. 13–33. Austrian Academy of Sciences Press, Vienna.) plus the current sheet model (Connerney, J. E. P., M. H. Acuña, and N. F. Ness 1981.J. Geophys. Res.86, 8370–8384.). Limb positioning errors, photometric uncertainties, atmospheric seeing changes, telescope point spread functions, and disk emissions are all included in the model. A generalized inverse methodology, applied to individual pixel intensities in the sequence of auroral images, is used to constrain the model parameters. A uniformly bright auroral oval, and two non-uniform emission models were examined; one has an anomaly fixed to the System-III coordinate, and the other has an anomaly fixed in planet's local time. The latter has a broad and bright emission in the afternoon sector, and offers a significantly better fit in the south. Finally, a combination of the System-III fixed anomaly (equatorward of the oval) plus the local-time fixed anomaly (poleward) was found to simulate the emission intensities in both aurorae consistently well. The size of the auroral oval is larger in the north (by 10%) and smaller in the south (by 3–7%) compared to the O6model auroral oval. Weaker H+3emissions are found both poleward and equatorward of the oval, the latter extending approximately to the footprint of the Io torus at most longitudes. System-III anomalies are tentatively identified at longitudes where the local magnetic field strength is less than that at the conjugate point.

High‐latitude distributions of plasma waves and spatial irregularities from DE 2 alternating current electric field observations

An 18‐month data base from the Dynamics Explorer 2 AC electric field spectrometers is used to obtain average high‐latitude magnetic local time (MLT) versus invariant latitude (INL) distributions of signal intensities in 12 frequency bands between 4 Hz and 512 kHz. Three distinctly different distributions are obtained, corresponding to (1) Doppler‐shifted signals from spatial structures in the electric field (i.e., irregularities) and Alfven waves between 4 and 512 Hz, (2) ELF waves between 256 Hz and 4.1 kHz, and (3) VLF waves between 4.1 and 64 kHz with extensions into the 128–512 kHz band. The ELF and VLF distributions closely resemble previously published results based on more limited sampling. Comparable distributions for the seven channels between 4 and 512 Hz, showing a prominent zone of maximum intensities at 72.5°–80° INL between 0500 and 1300 MLT, have not previously been reported. The power law frequency dependence of average power spectral densities (PSDs) between 4 and 512 Hz is also mapped in MLT‐INL coordinates. At all locations, two power law indices (slopes) are required to closely fit the PSDs with an inverted knee joining the two slopes in the 32–64 Hz band. This knee band corresponds to the range of O+ cyclotron frequencies encountered, and it lends credence to Gurnett et al.'s (1984) contention that Alfven waves are an essential ingredient in explaining the low‐frequency in situ satellite signals which were previously attributed to polarization fields accompanying spatial irregularities in plasma densities. However, other aspects of the 4–512 Hz observations, including seasonal variations, favor the earlier spatial irregularity interpretation. As discussed, the difficulties encountered in seeking interpretations exclusively in terms of either spatial irregularities or Alfven waves can be resolved with a synthesis approach requiring both types of signals. It is proposed that the averaged intensities and corresponding spectral characteristics in the 4–512 Hz band represent the consequence of intermittently superimposing shear Alfven waves on a spatially irregular medium. There are then three principal contributions: (1) an omnipresent 4–512 Hz signal from Doppler‐shifted responses to 2000–15 m spatial irregularities having an average power law spectral index near −1.9, (2) intermittent signals from locally generated shear Alfven waves having maximum power at frequencies of <4 Hz and average power law spectral indices of ≤(−2.8) extending only to fc(O+), and (3) spatial irregularity modulations of shear Alfven waves originating both locally and in the distant magnetosphere.

Pinhole x-ray cameras for imaging small-scaled auroral structures

A balloon-borne x-ray pinhole camera has been developed to detect and image auroral x rays in the energy range 20 to 120 keV. We have flown this camera on four different occasions and have imaged bremsstrahlung x rays from precipitated energetic electrons in the auroral zone and from a circumpolar navigating balloon in Antarctica. The images, which include several dynamic precipitation temporal forms, show that auroral x rays persistently include small spatial structures of ≈2O km (at ionospheric heights). The x-ray energy spectral information as a function of space and time shows that electron precipitation often includes two energy components that can be fit by exponential functions. Typical e-folding energies of these spectra are a few kilo-electron-volts and several tens of kilo-electron-volts. The x-ray camera remote senses characteristics of energetic electron sources in the distant magnetosphere that cannot be achieved by any other means.

Ultraviolet imaging of the Jovian aurora with the Hubble Space Telescope

We present here for the first time a Lyman‐α image of the north polar region of Jupiter obtained with the Faint Object Camera (FOC) on board the Hubble Space Telescope a few hours after the encounter of the ULYSSES spacecraft with Jupiter. The presence of high latitude regions of enhanced emission is clearly observed. A comparison with the location of the “UVS oval”, the Io (L = 6) and high‐latitude field‐line footprints shows that the best agreement is obtained with the L ≥ 15 footprint and the UVS oval which are close to each other for the particular longitudinal sector (30° < λIII < 210°) observed. These two L‐shells correspond to two possible sources of precipitation: particles originating respectively from the region of the plasma torus of Io in a distorted magnetic field or particles from the distant magnetosphere by analogy with the terrestrial aurora. The first direct determination of the latitudinal extent of the oval and of its intensity is made and compared with previous estimates.

Controlled experiments in the earth's magnetosphere with artifical electron beams

This article describes the Electron Echo project, which used artificial electron beams to determine how natural particles are accelerated and lost from the trapping region in the earth's magnetosphere. The problems of injecting electron beams into the ionosphere, including vehicle charging and beam-plasma instabilities, are discussed, and images of the Echo beams both in space and in the laboratory are shown. The beam pulses were detected and analyzed after reflection from the conjugate hemisphere mirror points. Although the injected beams contained a wide spread of energies, the echoes displayed a monoenergetic peak, which together with the exact drift displacement during one bounce permitted a separate evaluation of electric and magnetic components of the transverse drifts in the distant magnetosphere. Thus distant electric fields were measured and found to have large turbulent fluctuations compared to the local ionospheric fields, and to differ in average value from the local fields by about 50 mV/m, indicating the existence of parallel potential drops. The measured bounce times and field-line lengths were compared with model magnetic geometries. The best fit was with the Tsyganenko-Usmanov 1982 version. Pitch-angle diffusion out of the loss cone resulted in a large loss in beam intensity during one bounce, showing the presence of important beam perturbations near the electron gyrofrequency range. Thus the Echo experiments evaluated many effects on the natural trapped particles, but were limited to a moderately disturbed condition of the magnetosphere when the magnetic morphology was stable.

Electron beam sounding rocket experiments for probing the distant magnetosphere.

Electron accelerators on sounding rockets have injected 8-40-keV electrons on closed magnetospheric tail field lines near 250 km altitude in the northern auroral zone. These beams mirrored at the southern conjugate point ad returned as 'echoes' which were detected on the rocket system. The 20 percent of the beam that returned was sufficient to measure field line lengths and verify magnetospheric magnetic models, to measure fluctuating electric fields, and electron pitch angle scattering (6-10) R(E) distant, and to identify 10-100 V field-aligned potentials above the rocket. The experiment gives new insight into the motion of natural electrons in the outer Van Allen radiation belt.

Global dynamics of the magnetosphere for northward IMF conditions

Ground-based and spacecraft observations of polar cap geophysical phenomena during periods of northward interplanetary magnetic field (IMF) show specific patterns of electric fields, field-aligned currents, aurora and particle precipitation. These are basically different from those when the IMF is southward. The total combination of observational data for northward IMF indicates rather a closed magnetosphere. This topology has led to the formation of a specific convection pattern in the distant plasma sheet. As different theoretical studies show, the connection of the IMF to geomagnetic flux tubes poleward of the cusp region may serve as the driving mechanism for plasma sheet convection and as the dynamo of current systems. Unfortunately, the direct observations of processes in the distant magnetosphere are too scarce either to accept or reject the concept of a closed magnetosphere. There are also some experimental data that are inconsistent with the closed magnetosphere topology. Definitive open or closed models must await future measurements.

The turbulence of Alfven waves in the polar magnetosphere of the earth

Effects of nonlinear interaction of sheared Alfven waves providing the energy transport from the distant magnetosphere to the auroral ionosphere are discussed. The decay interaction occurs both in a warm [1 ≫ β > ( m e m i )] plasma and in a cold [β < ( m e m i )] plasma. The growth rate of the decay instability is sufficiently large and Alfven wave turbulence is excited. The excitation of convective plasma vortices and stationary field-aligned currents is also possible at high altitudes ( h ≳ 3 R E ). In this case, the turbulence of Alfven waves becomes strong and resembles two-dimensional MHD-turbulence, characterized by power-law spectra of magnetic and electric fields. A comparison with satellite data is made.

Investigation of precipitating electrons during an auroral breakup

We have studied the detailed electron distributions measured in a westward travelling surge. The particle data, obtained by a sounding rocket, have been related to the auroral structures as recorded by a low light level TV. The event took place close to magnetic midnight on January 27, 1979.Two different types of particle spectra were detected. The first type consisted of two populations; a low energy population of secondary electrons with j ∼ E−2 and a plasma sheet population. The second type of spectra contained a Maxwellian population with a characteristic energy of a few hundred eV in addition to the previous two populations. The few hundred eV Maxwellian and the plasma sheet population had been accelerated in a potential drop parallel to the magnetic field lines.The auroral luminosity depends on the integrated energy flux. In regions with medium or low luminosity the spectra could be of either type, while the brightest auroral structures were connected to spectra of the second type, that is accelerated spectra with a low temperature population. It is suggested that the parallel potential drops are created on field lines magnetically connected to regions in the distant magnetosphere where the few hundred eV population is present.

On the generation of quasi-electrostatic half-electron-gyrofrequency VLF emissions in the magnetosphere

A theoretical study is made of the generation mechanism of quasi-electrostatic VLF emissions observed in the distant magnetosphere, with frequency greater than half the electron gyrofrequency and with wave normal around the cold plasma oblique resonance cone. The two-component plasma is treated, composed of cold electrons and hot electrons with bi-Maxwellian and loss-cone distribution functions. The effects of various plasma parameters on the instability characteristics are examined in order to estimate their relative importance in determining the properties of unstable waves. It is found that both types of hot plasma distribution function can account for quasi-electrostatic half-gyrofrequency emissions. The frequency where the maximum growth rate occurs is mainly determined by the temperature anisotropy of the hot plasma, and the wave normal angle where maximum growth is expected is determined by the temperature of the hot plasma and the ratio of the cold to hot plasma densities. These theoretical considerations form the basis of a suggested plasma model which is able to explain our experimental direction-finding results.

Generation of magnetic-field aligned currents, parallel electric fields, and inverted-V structures by plasma pressure inhomogeneities in the magnetosphere

Magnetic-field aligned currents driven by plasma pressure inhomogeneities (plasma clouds) in the distant magnetosphere are analyzed quantitatively. A parallel potential drop is found to be established in the upward current region whenever a spatial scale D 0 for the pressure gradient in the equatorial magnetosphere is smaller than ≈ 3g 0 B i B 0 , where g 0 is a hot electron gyroradius in the equatorial magnetic field B 0 ( B i denotes the magnetic induction in the ionosphere). A theoretical derivation is given for the experimentally observed linear relation T = AE p + T 0 between the characteristic energy T of precipitating magnetospheric electrons and the peak energy E p in inverted-V electron spectra. Three-dimensional potential structures accelerating electrons earthward are shown to be established beneath some model clouds which could correspond to a large scale inverted-V structure and to a thin (~ 1 km) auroral arc.